In modern engines fuel is injected by means of a fuel injection valve or an injector directly into the cylinders of an engine. Since the injection occurs at a relatively late phase at the end part of the compression stroke, a sufficiently high pressure is required for the injection. In a conventional fuel feeding system, each cylinder is provided with an injection pump of its own which pumps fuel through an injection valve and an injection nozzle into the combustion chamber of the cylinder However, the use and control of the conventional system has significant limitations. The settings of the system cannot be adjusted easily. In addition the pressure in the injection pumps may vary, so that the injection into the different cylinders may take place under different pressures and may thus provide different amounts of fuel, respectively. Also, since the injection nozzles of prior art have been predominantly hydromechanical, i.e. opening at a certain predetermined fuel pressure and closing when the pressure decreases below the predetermined value, the control of the injection timing and duration should be able to take into account the wearing of the system components also during the use of the system, i.e. when the engine is running.
A more recent solution is the so called “common rail injection” or “common pressure injection”, in which the provision of pressure and the injection of fuel are functionally separated from each other. Fuel is fed by means of at least one high pressure fuel pump into a common pressure supply, i.e. rail, from which it is led through separate pipes into the injector or injection valve of each cylinder. In practice, the operation of an injector is electronically controlled, for instance by means of a solenoid or piezoelectric valve, in order to obtain a sufficiently short and precise injection.
A number of the most obvious problem areas of traditional fuel feeding systems have been solved by the use of a high pressure (up to about 2200 bar) common fuel supply, and electronically controlled fuel injection valves by means of which it is, for instance, possible to inject fuel into an engine cylinder several times during the same compression stroke. In other words, the timing of the injection, the duration of the injection and the quantity of injected fuel is in clearly better control than with the fuel injection pumps of prior art, whereby also the emission levels in normal operating conditions of a piston engine have been drastically reduced.
This far the diesel engines have been optimized in view of their emissions at full load. However, the future emission legislation requires that the emission levels have to be minimized at all operating conditions. In other words, all loads spectrum tuning has to be performed. For instance, a case where even the use of modern common rail fuel system and electronic control of a fuel injection valve do not bring desired results relates to running an engine in low load, or, more generally, substantially far from its design load. The ultimate goal is to improve the injection of fuel such that the emissions of an engine throughout its operating conditions i.e. from low load to full load could be kept on minimal level.
This endeavour has led to the use of injection valves having two injection nozzles. For instance, U.S. Pat. No. 7,556,017 B2 discusses a fuel injector having an injector body defining a hollow interior configured to receive pressurized fuel, a first nozzle configured for providing a first fuel spray pattern, and a second nozzle configured for providing a second fuel spray pattern different from the first fuel spray pattern. The first and second nozzles may be configured to inject fuel supplied from a common source into a combustion space. The nozzles may be used in separate stages during the compression stroke of piston such that the first nozzle injects a predetermined amount of fuel in an early stage of the compression stroke, and the second nozzle at a later stage or at the end of the compression stroke.
U.S. Pat. No. 6,422,199 B1 discusses a fuel injector having a nozzle body with two valve needles in side-by-side configuration. The document discloses various alternatives for the use of the valve needles whereby it is possible to choose from which outlet opening to inject fuel, for how long and at which time. Further it has also been disclosed that both outlet openings may be open simultaneously.
Prior art, EP-A1-0 972 932, knows also fuel injection valves having two sets of injection openings. When lifting an injection needle member a small amount a first set of openings open and inject into the engine cylinder a small amount of fuel needed in low load operations. When the injection needle member is lifted further another set of openings is opened and more fuel is injected into the cylinder corresponding to full load operation. JP-A-62118055 teaches another injection valve structure comprising two sets of injection openings that can be opened separately. The two sets of injection openings are in flow communication with two injection valves arranged side by side in the same injector holder.
Also, injection valves having two injection needle members one inside the other have been discussed. DE-A1-10 2007 000 037, DE-B4-10 2007 000 095 and DE-C2-44 32 686 may be mentioned as examples of such fuel injection valve structures.
The use of an injection valve having a twin-needle structure as well as the use of two injection valves per each engine cylinder complicates the electronics and requires additional fuel lines and wiring in the surroundings of the cylinder head by doubling the components required for the fuel injection. In other words, each injection nozzle requires a control valve, a high-pressure fuel line from the common rail to the injection valve, a flow fuse in the high pressure fuel line, a low pressure fuel line for returning fuel and a wiring for the solenoid of the control valve. The flow fuse is a component installed between the common rail and the injection valve for detecting changes in flow pressure. For example, the flow fuse stops the feeding of fuel if the injection needle in the injection valve gets stuck so that it will not totally close i.e. the flow fuse prevent the injector from bleeding fuel into the cylinder continuously. Now that in modern engines there are two intake valves and two exhaust valves i.e. four valves per cylinder, the top surface of the cylinder head has to house these four valve stems and valve springs. Additionally, there are at least mounting blocks for the rocker shaft attached to the cylinder head, and sometimes also openings for the pushrods operating the rocker arms. And finally, in case it is a question of a dual-fuel engine there are means for admitting natural gas to the cylinder arranged to the top of the cylinder head. Thus, the space for the additional components a second fuel injection valve requires is very limited leading to structures that complicate the installation and maintenance work of the components at the cylinder head.
Another case where additional attention has to be paid to the emissions, and to the operating of the fuel injection valves relate to dual-fuel or tri-fuel engines i.e. engines that use both natural gas and light fuel oil or heavy fuel oil. The use of two injection valves, normally one smaller and another larger injection valve, per cylinder is already known in the dual-fuel engines. In normal continuous operation the natural gas is the main fuel, which is ignited in the engine cylinder by means of pilot fuel injected by means of the smaller injection valve. The larger injection valve finds its use most often in the starting phase of the engine, and it is used until the combustion is stable in each cylinder whereafter the gas admission may be started. Another, seldom occurring use of the larger injection valve is when the gas admission into the cylinders does not, for some reason, function properly. In such a case the larger injection valve is used to feed so called backup fuel into the cylinders. It is normal practice that the pilot fuel feeding system utilizes the common rail fuel feeding system. However, the backup fuel feeding system is the traditional injection system where for each cylinder, and each pump line nozzle, there is a jerk pump, and a pressure line from the jerk pump to the nozzle. In such a system each stroke of the jerk pump opens the pump line nozzleand injects a certain amount of fuel into the cylinder. Often, the fuel systems of the pilot fuel and the backup fuel are separate starting already from two different fuel tanks.
What the existence of the above-discussed fuel systems of a dual-fuel engine mean in practice is that, for each cylinder of an engine, the engine has to be provided, on the one hand, with the components required by the common rail system of the pilot fuel i.e. a flow fuse, a high pressure fuel line from the common rail fuel supply to the flow fuse, a control valve, high pressure fuel lines from the flow fuse to the injection valve, the injection valve itself and the low pressure fuel line from the injection valve to the low pressure fluid reservoir, and, on the other hand, components of the backup fuel system, i.e. the fuel injection pump for creating the injection pressure, the fuel line between the fuel injection pump and the injection valve, return fuel line from the fuel injection pump to the low pressure fuel reservoir and the injection valve. Thus it is clear that the room between the valve springs above the cylinder head is filled with different components, whereby both the installation and maintenance of the components is complicated.
Also, as already discussed above the conventional fuel injection system is susceptible to wear, which results in various problems in the use thereof. Expenses and risks involved in the use of injection pumps form yet another problem that requires attention. Naturally also the use of two liquid fuel systems side-by-side increases both expenses, risks of malfunction and space required by the systems.
An object of the present invention is to design the injection valve structure such that the number of components that have to be positioned in the surroundings of the cylinder head and the engine is reduced in comparison to prior art structures.
Another object of the present invention is to lower the emissions involved in the use of diesel fuel in dual-fuel engines and enable fast switching from pilot to main diesel mode.
Yet another object of the present invention is to lower the expenses involved in the use of diesel backup fuel in dual-fuel engines.
Still another object of the present invention is to minimize the risks of component failure involved in the use of diesel backup fuel in dual-fuel engines.